1. Field of the Invention
The present invention relates to a fine powder of metallic copper and process for producing the same, more particularly a fine powder of metallic copper, suitable as a material for electroconductive pastes, and having a BET diameter of 3 μm or less, large crystallite size, high dispersibility and particles of high sphericity, and a process for producing the same.
2. Description of the Prior Art
An electroconductive metallic powder for electroconductive pastes to be used for forming circuits or multilayer capacitors is required to be low in impurity content, and have particles uniform in shape and size, and well dispersed while being little agglomerated, among others. The other requirements include high dispersibility in the paste and high crystallinity to prevent uneven sintering.
More specifically, the metallic powders have been particularly demanded recently to be composed of particles having:    (1) a size determined by the BET method (hereinafter sometimes referred to as BET diameter) of 3 μm or less,    (2) highly spherical shape and high dispersibility, and    (3) a sufficiently large crystallite size to prevent reoxidation.
One of the well-known processes for producing fine metallic powders is gas spraying, in which molten metal is sprayed from one or more nozzles into an inert gas, e.g., argon, to be quenched therein. However, it is difficult for such a process to produce particles of high sphericity and uniform size, 3 μm or less. When particles of high sphericity having a size of 3 μm or less are to be produced by this process, it is necessary to classify the spherical particles produced, which decreases the yield and pushes up the cost. Another problem involved in this process is observed when spherical particles of base metal, e.g., copper, are to be produced, because they are oxidized while the molten metal is sprayed to only give a product of high oxygen content.
Another known process for producing fine metallic particles is spray pyrolysis, in which a solution or suspension of one or more types of metallic compounds is sprayed into fine droplets and thermally treats them to decompose the metallic compound at a temperature level higher than its decomposition temperature, preferably close to or higher than its melting point, in order to separate out the powdery metal or alloy (see e.g., JP-B-63-31522).
This process can give highly spherical particles of metal or alloy, high in crystallinity, or single-crystalline and high both in density and dispersibility. It has several advantages. For example, it needs no solid/liquid separation, unlike the wet reduction process to simplify the production process, and also needs no additive or solvent which may affect the product purity, to give the high-purity powder free of impurities. Moreover, it can easily control particle size, and also easily controls the product composition, because composition of the product particles basically coincides with that of the metallic compound(s) in the starting solution.
However, this process involves a problem: it thermally decomposes droplets containing the starting metallic compound(s), which invariably decomposes the solvent, e.g., water, or alcohol, acetone, ether or another organic compound, to increase the energy cost for the pyrolysis or the like.
This process evaporates the solvent under heating and then thermally decomposes the particles of the condensed metallic compound(s), which needs a large quantity of energy for evaporating the solvent. Moreover, the product powder may have a broader particle size distribution, when the sprayed droplets coalesce with each other or are broken up. Prevention of these problems needs fine control of the reaction conditions, e.g., spraying speed, concentration of the droplets in the carrier gas and residence time in the reactor, which is very difficult to realize. Moreover, this process, when applied to production of powder of base metal, e.g., copper, needs a reducing or weakly reducing atmosphere under which the thermal decomposition is strictly controlled, which is difficult. Still more, when water is used as the solvent, the oxidative gas generated by decomposition of water oxidizes copper or the like, with the result that the powder of high crystallinity can be no longer obtained.
A vapor-phase chemical reaction process is also well-known for producing metallic particles. For example, there is a process which reacts cuprous chloride vapor with a reducing gas at 700 to 900° C. to produce the fine copper particles (see e.g., JP-A-2-57623).
In this process, cuprous chloride vapor, evaporated at 700 to 900° C., is reacted with hydrogen to produce fine copper particles having large crystallite size and resistant to oxidation.
In this process, however, production rate of the fine copper particles is determined by the vapor pressure of cuprous chloride at 700 to 900° C., and hence is limited. Therefore, the process has a disadvantage of being difficult to have a high production rate and hence high production capacity. Moreover, the particles separated from the vapor phase tend to agglomerate with each other and are difficult to control particle size.
One of the processes which have been recently proposed is reduction based on solid/vapor reaction, in which powdered metallic compound, e.g., tungsten oxide, is brought into contact with a gaseous reducing agent (see e.g., JP-A-11-503205). More specifically, the powdered metallic compound to be reduced is sprayed, together with the gaseous reducing medium and carrier gas, into a temperature-controllable reaction chamber, where the powdered metallic compound is passed through the reaction zone in a given track for 0.4 to 60 seconds on the average, to reduce the compound to a conversion of 90% or higher.
This process, initiating the reaction itself by bringing the solid starting compound into contact with the reducing gas, involves a problem of being difficult to completely reduce the starting compound into the metallic state in a short time, because it has a smaller reaction area than the vapor-phase process described above. Moreover, it is difficult for this process to completely reduce the starting compound into the metallic state, even when the reaction time is extended by use of a cyclone as the reaction vessel to extend the particle tracks or by breaking up the solid starting compound to reduce its size and thereby to increase its reaction area. Therefore, this process is considered to be difficult to produce high-crystallinity, particles of high sphericity and uniform size, suitable for electronic devices.
More recently, another process is proposed, in which one or more types of thermally decomposable metallic compound powders, e.g., metallic hydroxide, metallic nitrate or organometallic compound, are charged into a reactor together with a carrier gas, dispersed in the vapor phase at a concentration of 10 g/L or less, and heated at the decomposition temperature or higher but (Tm-200)° C. or lower (Tm: melting point of the metallic compound) (see e.g., JP-A-2002-20809).
This process keeps the reaction atmosphere reductive, irrespective of carrier gas, by use of an organometallic compound as the starting material, although the metal is base in itself, to produce the metallic particles.
However, it is essential for this process to use starting particles of uniform size, because size of the product metallic particles is in proportion to that of the starting particles. The starting particles, therefore, should be crushed, beaten or classified beforehand by a crusher or classifier. Moreover, an organometallic compound, when used, should be completely combusted, which additionally increases the energy cost. Still more, this process tends to form an oxide, nitride or carbide.
As discussed above, fine metallic powders suitable for electroconductive pastes have been strongly in demand, as demands for electroconductive metallic powders are rapidly growing for circuits and laminate condensers. However, the conventional fine base metal powders, in particular copper, cannot satisfy these requirements simultaneously. Therefore, there are strong demands for fine powders of metallic copper, having a BET diameter of 3 μm or less, large crystallite size, high dispersibility and particles of high sphericity.